Halogenated aromatic polyesters such as the condensation products of 4,4'-isopropylidene-2,2',6,6'-tetrachlorodiphenol or 4,4'-isopropylidene-2,2',6,6'-tetrabromodiphenol with isophthalic acid and/or terephthalic acid or the ester-forming derivatives thereof have been used to produce a number of inherently non-burning fibrous materials which are particularly advantageous when fibrous articles are required for use in fire-control environments, such as children's sleepwear, suits for fire fighters, hospital furnishings, and uniforms for military and civilian personnel. However, these halogenated aromatic polyesters are sensitive to organic solvents such as perchloroethylene, which is a commercial dry cleaning solvent.
Copending patent application U.S. Ser. No. 208,364, entitled "Improved Process for Drawing Halogenated Aromatic Polyester Fibers", filed concurrently herewith by Albert G. Williams, discloses a process for drawing halogenated aromatic polyester fibers at a temperature of from about 315.degree. to about 355.degree. C. at a draw ratio of from about 3:1 to about 10:1 to produce halogenated aromatic polyester fibers having improved tensile properties. To some extent, this drawing process also improves the solvent resistance of the fibers. However, some solvent sensitivity of present even after the halogenated aromatic polyester is drawn at these high draw ratios and elevated temperatures.
U.S. Pat. No. 3,684,766 and U.S. Pat. No. 3,780,148 describe a process for crystallizing a low molecular weight polyester with non-halogenated crystallizing agents prior to a second stage reaction.
In copending patent application U.S. Ser. No. 208,363, entitled "Solvent Resistant Halogenated Aromatic Polyester Fibers And Process Therefor" and filed concurrently herewith by Albert G. Williams, a process is disclosed for improving the solvent resistance of halogenated aromatic polyester fibers. This process comprises heat treating the halogenated aromatic polyester fibers at a temperature of from about 270.degree. to about 295.degree. C. for from about 5 to about 60 minutes to produce modified halogenated aromatic polyester fibers capable of withstanding 5 to 20 minute immersions in perchloroethylene which is at a temperature of from about 60.degree. to about 70.degree. C. Although this process improves the solvent resistance of halogenated aromatic polyesters such that they are resistant to dry cleaning solvents under relatively mild conditions, the search has continued for methods for improving even further the solvent resistance of halogenated aromatic polyesters. This invention was made as a result of that search.